The present invention relates to production logging systems for use in the oil and gas industry; more particularly, it relates to a method and apparatus for conducting radioactive tracer logging to determine the injection profile of a well.
To enhance hydrocarbon production, fluids are often injected into a subterranean formation to urge hydrocarbons to producing wells. Such operations are referred to as "secondary" or "tertiary" recovery techniques. Wellbores into which the fluids are injected are referred to as "injection wells". Optimization of fluid injection recovery techniques requires that the fluids be injected at depths where formation permeability is greatest. Accordingly, a profile of the depth locations and relative quantities of fluid exiting the injection wellbore, referred to as a "injection profile", is advantageous. Radioactive tracer logs are commonly used to determine the injection profile in water injection wells.
In radioactive tracer logging, a small quantity of radioactive material is dissolved in a carrier and injected downhole into the fluids flowing in the tubing or casing. The distribution of the radioactive tracer is monitored by a gamma ray detector. There are primarily two radioactive tracer logging methods in use for determining injection profiles. These methods are the velocity shot method and the tracer loss method.
In the velocity shot method, a single slug of tracer material is ejected from a downhole logging tool. Two gamma ray detectors mounted on the tool and spaced a discrete distance apart measure the passage of the tracer slug. The velocity in the wellbore at a particular location is determined from the transit time of the radioactive tracer between the two detectors. The accuracy of the velocity shot method is based on the assumption that the velocity is constant betwen the two detectors. The assumption is not valid, however, if the tool is opposite a part of the formation that is receiving injected fluid or if the wellbore diameter is not constant. The fallacy of the basic assumptions produces errors in the injection profile.
The tracer loss method involves injecting a single slug of tracer material into the wellbore at a location above all fluid injection. The tracer slug is followed as it progresses down the borehole by repeatedly passing through the tracer slug a gamma ray detector mounted on the logging tool. The repeated detection yields a series of plots of tracer concentration verses depth. The tracer loss method assumes that the decrease in total amount of tracer in the wellbore is proportional to the amount of fluid exiting the wellbore above the measurement location. However, since gamma rays are able to penetrate into a formation, at least some of the gamma ray signal assumed to be due to tracer in the wellbore may be originating from tracer material that has entered the formation. Additionally, there is significant dispersion of the tracer slug such that the depth locations of fluid loss to the formation cannot be accurately ascertained.
Other radioactive tracer logging techniques have been proposed. For example, U.S. Pat. No. 3,590,923 to Cooke, Jr. discloses a method in which two tracers of different partition coefficients are injected into a formation. The degree of separation is measured to determine fluid saturations. The method is based on chromatographic analytical techniques. Similarly U.S Pat. No. 4,168,746 to Sheely teaches to inject a plurality of tracers at different partition coefficients at different distances into a formation for evaluation of surfactant flood. This method is also a chromatographic technique. Finally, U.S. Pat. No. 4,055,399 to Parrish discloses a method for testing formation velocity in which a different one of a plurality of different tracers is added to each of a number of fluid slugs. At least two of the tracers are added in different pre-selected ratios, and an analysis is made of recovered fluid to measure the content of each of the tracers and thereby determine which of the fluid slugs produced the recovered fluid.
The present invention provides an improved radioactive tracer logging method which is more accurate than methods heretofore used.